This invention relates to multilayer semiconductor structures, and, more particularly, to a semiconductor structure having a buffer structure between the active material and the substrate.
A common construction used in a semiconductor device is an active semiconductor material supported upon a substrate of another material. The compositions of the active material and the substrate are selected for the particular purpose of the device, and a wide variety of combinations are possible. For example, the active material is often a doped II-VI or III-VI semiconductor compound, and the substrate is often silicon or gallium arsenide. The active material and the substrate may also be multilayered structures, chosen to achieve particular functions.
The crystalline relations and defect structures of the active material and the substrate are important factors in the operability of the semiconductor device. The active material and the substrate have inherent defect structures that are produced during their growth or deposition. The active material is usually deposited so as to be epitaxially related to the substrate material at their interface. There is therefore continuity of lattice planes across the interface between the active layer and the substrate. However, because the lattice parameters of the active material and the substrate are not usually the same, the epitaxial relation results in a state of strain being present at and adjacent to the interface. Differences in thermal expansion coefficients between adjacent phases produce similar states of strain if the structure is heated or cooled during fabrication or service.
The strain state at and near the interface can affect the operation of the semiconductor device. Additionally, the strain state can sometimes be relieved by the production of defects at the interface. The defects inherently present in the constituents and those produced due to the epitaxially induced strain state can adversely affect the performance of the semiconductor device.
It is therefore usually desirable to minimize the strain state and unwanted defect structure in the semiconductor device. One approach to achieving this end is to place a buffer layer between the active material and the substrate. The buffer layer provides epitaxial continuity, but has a lattice parameter intermediate those of the active material and the substrate. Strain and interfacial defects are thereby reduced.
For example, in one type of infrared sensor it is desired to utilize HgCdTe active material epitaxially upon a silicon substrate. HgCdTe has a lattice parameter of 6.46 Angstroms for compositions typically used in infrared detectors, and silicon has a lattice parameter of 5.43 Angstroms. It is not yet possible to deposit the HgCdTe directly onto the silicon to achieve an epitaxial relation because of this large lattice mismatch, and when the deposition is successful there is necessarily a large mismatch strain and a high associated defect density.
To alleviate this problem, it has been the conventional practice to use a gallium arsenide buffer layer between the HgCdTe active material and the substrate. There are, however, several significant drawbacks to this approach. There remains a high defect density. Moreover, if the HgCdTe is deposited by the liquid phase epitaxy technique, the gallium arsenide can dissolve into and contaminate the liquid phase.
An additional CdZnTe layer can be added to the buffer structure between the HgCdTe active material and the gallium arsenide. The CdZnTe layer can block the penetration of defects from the gallium arsenide into the HgCdTe active material. Semiconductor devices made with this substrate/buffer layer/active material structure work well in some applications. However, the defect structure remains excessive for optimal performance of the semiconductor device in other applications. Additionally, there is always the concern with contamination of the liquid phase epitaxy melt, when that approach is used.
There remains a need for an improved approach to the construction of semiconductor devices of the type discussed. The approach should overcome the problems identified in the prior approaches. The present invention fulfills this need, and further provides related advantages.